This disclosure relates generally to oil and gas well logging tools. More particularly, this disclosure relates to tools for measuring rock formation porosity through the use of gamma ray signals created when neutrons emitted by a pulsed neutron source interact with the materials surrounding the tool and with the material of the tool itself. This disclosure relates to a neutron porosity tool that uses a single gamma ray detector covered with neutron absorbing material like B10 isotope for the porosity measurements.
In petroleum and hydrocarbon production, it may be desirable to know the porosity of the subterranean formation that contains the hydrocarbon reserves. Knowledge of porosity may be used in calculating the oil saturation and thus the volume of oil in-place within the reservoir. Knowledge of accurate porosity values is particularly useful in older oil wells where porosity information derived from open hole porosity logs is either insufficient or nonexistent. This porosity information is useful in determining remaining oil in-place and in determining whether the remaining oil in-place justifies applying enhanced recovery methods. Porosity information is also helpful in identifying up-hole gas zones and differentiating between low porosity liquid and gas.
A variety of tools exists which allow the porosity of the reservoir to be determined. Most of these tools are effective in determining the porosity of the reservoir around the wellbore in which the tool is run. There are devices that work with cased holes as well as uncased holes. A common feature of these prior art tools is the necessity of having at least two neutron detectors. The present disclosure is directed towards a tool in which a single gamma ray detector is used for estimating formation porosity.